How to dry your hands.

I talked about how when you really understand something, the possibilities for lateral thinking really emerge.

Drying hands.

So there I am one day, in a motorway service station, having been to the toilet and washed my hands. I step over to the dryer, and put my hands under it and feel the warm air blow over them.

I've done this many times in my life, but this time I stopped and thought about it. What was I trying to do? I was trying to get my hands dry.

OK - that's the simplistic question, now lets see if I can refine it a bit more, what am I trying to do? I'm trying to get the water that is on my hands to evaporate into the air.

Evaporation is the key.

Now how does that come about? Well lets think about it a bit. The water on my hands is a collection of H₂O molecules that are attracted together by weak forces. On average, if you add up all the energy of the molecules and divide by the number, you get a value for the average energy, which is what we call heat. Some molecules have this energy, some have more, some have less.

Now and again, these forces are weak enough that a molecule near the surface which happens to have more than the average amount of energy breaks away from the pack and flys free. Once it is free, it gets to wander around in the air. As a result, the average energy drops a tiny bit, and so therefore does the heat. That's why wet hands feel cold.

Escape to freedom?

However there is a reasonable chance that any given escaping molecule may bounce off some other air molecules, and end up back in the bit of water its just left. It may not, but if might. You can reduce this by moving the air around just above the water - a hand dryer does this - OK so that's good.

Encouraging the leap

How can I encourage water molecules to make the leap from liquid to gas? Well its all down to their energy. If they have enough, and they're near the surface there is a good chance they'll break free. Give them more energy and the chances will increase. I supply them with energy already. My body heat supplies the water molecules vibrational energy.

I can't do much about my body temperature. Evolution has worked long and hard to give me a body that maintains a constant temperature. However, I can get more heat into the water from the hot air coming out of the drier. So holding my hands under the warm air is going to give more energy to the water molecules.

Making the leap easier.

OK - well I was already doing that. Is there anything else I could do?
Yes there is. These evaporating high speed molecules can only get out if they are near the surface of the water. If they're not, chances are they'll bounce into some other molecule on the way out and never make it. So if I can make the surface bigger, and the layer of water thinner, I should be able to dry my hands quicker.

How can I do this? Well smearing the water out into thin layers is the easiest. A sphere of water has the least surface area for its volume, and a 1 atom thick film of water the most. So therefore the ideal way to dry my hands is to rub them together, so smearing out any drops of water, whilst holding them under the warm moving air. I need to balance the moving warm air getting into contact with the water with the shielding effect of my rubbing hands. So a vigorous action that keeps allowing the air to get at the water is best.

Result!

What do you know - that's just what it says on the dryer! However I feel so much more superior having worked it out for myself! Try it for yourself some day. Try just holding your hands under the dryer, and compare that to vigorously squashing the water flat by rubbing your hands together. See if you can spot the difference.